Fixes vif vRouter segment fault with bad arguments
Fixes vif vRouter segment fault with bad arguments

When vif is used with some invalid combinations of arguments it will
core dump with a segmentation fault. For example if vif -list is done
a core dump will happen. This is due to calling strtoul with a NULL
string. Adding a new procedure that will check if the string is NULL.

Change-Id: I5015c07e76a4434498229862f5172c2802ae99e9
Closes-Bug: #1590964

codilime/contrail-vrouter,codilime/contrail-vrouter,codilime/contrail-vrouter,codilime/contrail-vrouter